Display device and manufacturing method thereof

ABSTRACT

It is an object of the present invention to form a pixel electrode and a metal film using one resist mask in manufacturing a stacked structure by forming the metal film over the pixel electrode. A conductive film to be a pixel electrode and a metal film are stacked. A resist pattern having a thick region and a region thinner than the thick region is formed over the metal film using an exposure mask having a semi light-transmitting portion. The pixel electrode, and the metal film formed over part of the pixel electrode to be in contact therewith are formed using the resist pattern. Accordingly, a pixel electrode and a metal film can be formed using one resist mask.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/548,097, filed Oct. 10, 2006, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2005-301022 on Oct. 14, 2005, both of which are incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having a pixelelectrode, especially a display device.

2. Description of the Related Art

In manufacturing an active matrix display device, in general, a wiringto be connected to a semiconductor film of a thin film transistor (TFT)is formed and a conductive film to be a pixel electrode is formed overthe wiring. Therefore, a resist mask for forming the wiring and a resistmask for forming the pixel electrode have been required.

Further, as another example, an example is given, in which a conductivefilm to be connected to a semiconductor film of a TFT is formed, theconductive film is made to serve also as a pixel electrode, and a metalfilm is formed over the conductive film (for example, Patent Document 1:Japanese Published Patent Application No. Hei6-230425). This example isdifferent from the example described above. A transparent conductivefilm is employed as the conductive film, and the transparent conductivefilm is directly connected to the semiconductor film. The transparentconductive film is formed from a material having high resistance in manycases; therefore, in order to cover high electric resistance of thetransparent conductive film, the metal film is formed over thetransparent conductive film.

Also in Patent Document 1, in which the transparent conductive film isdirectly connected to the semiconductor film, a resist mask for forminga pixel electrode by etching the transparent conductive film and aresist mask for etching the metal film have been required.

In a conventional active matrix display device, a resist mask has beenrequired for each layer in forming a stacked wiring. In particular,there are a number of stacked structures in forming a pixel electrode,and at least a resist mask for forming the pixel electrode and a resistmask for etching a film to be stacked over the pixel electrode arerequired. Thus, there have been a number of manufacturing steps.Therefore, a manufacturing cost of a semiconductor device like a displaydevice has not been lowered.

SUMMARY OF THE INVENTION

In view of the foregoing problem, it is an object of the presentinvention to shorten a manufacturing process by forming a pixelelectrode and a film to be stacked over the pixel electrode using oneresist mask.

One feature of the present invention is to have a thin film transistorover a substrate, a pixel electrode electrically connected to the thinfilm transistor, and a metal film formed over the pixel electrode, wherethe metal film is in contact with the pixel electrode so as to cover alevel difference portion of the pixel electrode.

According to this structure, disconnection of a pixel electrode in alevel difference portion can be prevented. Disconnection refers that, byforming a film over a surface having a level difference portion, a crackis generated in the film in the level difference portion or part of thefilm is not formed owing to unfavorable coverage of the film in thelevel difference portion.

Another feature of the present invention is to have a thin filmtransistor over a substrate, a pixel electrode electrically connected tothe thin film transistor, and a metal film formed over the pixelelectrode to be in contact therewith, where the metal film has a smallerplane-surface area than the pixel electrode, a side of the metal film isalong a side of the pixel electrode, and the side of the metal film islocated inside the side of the pixel electrode.

According to this structure, the metal film can be used as part of alight-shielding film, and alignment of the light-shielding film can beeasily conducted.

One feature of the present invention is a display device having a thinfilm transistor over a substrate, a pixel electrode electricallyconnected to the thin film transistor, a metal film over part of thepixel electrode to be in contact therewith, a partition wall, which isformed over the pixel electrode and the metal film, and that makes partof the pixel electrode be exposed, an electroluminescent layer formed tobe in contact with the partition wall and the pixel electrode, and anelectrode over the electroluminescent layer, where at least one side ofthe metal film is inclined and covered with the partition wall.

According to this structure, short-circuit of a light emitting elementcan be prevented in an electroluminescent display device.

A pixel electrode and a metal film formed over part of the pixelelectrode to be in contact therewith can be formed using one resistpattern. Two patterns of the pixel electrode and the metal film can beformed using one resist pattern; therefore, a manufacturing process canbe shortened, and accordingly, a low-cost display device can berealized.

According to the present invention, the number of manufacturing stepscan be decreased more than before, and a manufacturing cost of asemiconductor device can be lowered. In addition, disconnection of apixel electrode in a level difference portion can be prevented because ametal film is formed over the pixel electrode to be in contacttherewith. An inexpensive and highly reliable display device with lessdisplay failure can be formed.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawings:

FIGS. 1A to 1C are cross-sectional views each showing a manufacturingprocess of a semiconductor device (Embodiment Mode 1);

FIGS. 2A to 2C are cross-sectional views each showing a manufacturingprocess of a semiconductor device (Embodiment Mode 1);

FIG. 3 is a top view of a semiconductor device (Embodiment Mode 1);

FIGS. 4A and 4B are cross-sectional views each showing a manufacturingprocess of a semiconductor device (Embodiment Mode 1);

FIGS. 5A to 5C are cross-sectional views each showing a manufacturingprocess of a semiconductor device (Embodiment Mode 2);

FIGS. 6A to 6C are cross-sectional views each showing a manufacturingprocess of a semiconductor device (Embodiment Mode 2);

FIG. 7 is a top view of a semiconductor device (Embodiment Mode 2);

FIG. 8 is a cross-sectional view showing a manufacturing process of asemiconductor device (Embodiment Mode 2);

FIGS. 9A to 9D are cross-sectional views each showing a manufacturingprocess of a semiconductor device (Embodiment Mode 2);

FIG. 10 is a cross-sectional view of a semiconductor device (EmbodimentMode 2);

FIGS. 11A to 11D are top views and a view showing light intensitydistribution of an exposure mask (Embodiment Mode 3);

FIGS. 12A and 12B are a top view and a cross-sectional view of an ELdisplay device (Embodiment Mode 4);

FIGS. 13A and 13B are a top view and a cross-sectional view of an ELdisplay device (Embodiment Mode 4);

FIGS. 14A and 14B are a top view and a cross-sectional view of a liquidcrystal display device (Embodiment Mode 5);

FIGS. 15A and 15B are a top view and a cross-sectional view of a liquidcrystal display device (Embodiment Mode 5);

FIGS. 16A and 16B are a top view and a cross-sectional view of a liquidcrystal display device (Embodiment Mode 5);

FIGS. 17A and 17B are a top view and a cross-sectional view of a liquidcrystal display device (Embodiment Mode 5);

FIG. 18 is a top view of a liquid crystal display device (EmbodimentMode 5);

FIG. 19 is a cross-sectional view of a semiconductor device (EmbodimentMode 1);

FIGS. 20A to 20D are views of electronic devices (Embodiment Mode 6);and

FIG. 21 is a view of a cellular phone (Embodiment Mode 6).

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiment modes according to the present invention will bedescribed. However, the present invention can be implemented in variousembodiments within the range of enablement, and it is easily understoodby those skilled in the art that various changes and modifications willbe apparent to those skilled in the art. Therefore, unless such changesand modifications depart from the scope of the invention, they should beconstrued as being included therein. In addition, embodiment modes shownbelow can be appropriately combined.

Embodiment Mode 1

In this embodiment mode, a method for forming a top-gate TFT over asubstrate 1 will be explained with reference to FIGS. 1A to 1C. Thesubstrate 1 is a substrate having a light-transmitting property, forexample, a quartz substrate, a glass substrate, or a plastic substrate.It is to be noted that the substrate 1 may be a substrate having alight-shielding property, and a semiconductor substrate or an SOI(Silicon on Insulator) substrate may be used.

An insulating film 2 is formed as a base film over the substrate 1. Asthe insulating film 2, a single layer of an insulating film such as asilicon oxide film, a silicon nitride film, or a silicon oxynitride film(SiO_(x)N_(y)), or a stack formed of at least two films of these filmsis used. Then, an island-like semiconductor film 3 is formed over theinsulating film 2.

The island-like semiconductor film 3 is formed in the following manner asemiconductor film is formed over the entire surface of the insulatingfilm 2 by a sputtering method, an LPCVD method, a plasma CVD method, orthe like, then, a shape of the semiconductor film is processed using amask formed by a photolithography method or the like. When theisland-like semiconductor film 3 is formed of a crystallinesemiconductor film, there are a method for forming a crystallinesemiconductor film directly over the substrate 1 and a method forforming a crystalline semiconductor film by crystallizing an amorphoussemiconductor film by heat treatment after forming the amorphoussemiconductor film over the substrate 1. In the latter method, heattreatment in crystallization is conducted by a heating furnace, laserirradiation, or irradiation of light emitted from a lamp instead oflaser light (hereinafter, referred to as lamp annealing), or acombination thereof.

In addition, the crystalline semiconductor film may be formed by athermal crystallization method conducting the above heat treatment afterdoping the amorphous semiconductor film with nickel or the like. It isto be noted that, in a case of obtaining the crystalline semiconductorfilm by crystallization using a thermal crystallization method with theuse of nickel, gettering treatment by which nickel is removed aftercrystallization is preferably conducted.

In a case of manufacturing the crystalline semiconductor film bycrystallization with laser irradiation, a continuous-wave laser beam (CWlaser beam) or a pulsed wave laser beam (pulsed laser beam) can be used.As the laser beam that can be used here, a beam oscillated from one ormore of gas lasers such as an Ar laser, a Kr laser, or an excimer laser;a laser using, as a medium, single crystalline YAG, YVO₄, forsterite(Mg₂SiO₄), YAlO₃, or GdVO₄, or polycrystalline (ceramic) YAG, Y₂O₃,YVO₄, YAlO₃, or GdVO₄ doped with one or more of Nd, Yb, Cr, Ti, Ho, Er,Tm, and Ta as a dopant; a glass laser; a ruby laser; an alexandritelaser; a Ti:sapphire laser; a copper vapor laser; and a gold vapor lasercan be used. A crystal with a large grain size can be obtained byirradiation of a laser beam having a fundamental wave of such laserbeams or one of second, third, and fourth harmonic of these fundamentalwaves. For instance, the second harmonic (532 nm) or the third harmonic(355 nm) of an Nd:YVO₄ laser (fundamental wave of 1,064 nm) can be used.This laser can be emitted with a CW or a pulsed wave. In a case ofemitting the laser with a CW, the power density of approximately 0.01 to100 MW/cm² (preferably, 0.1 to 10 MW/cm²) is required. The scanningspeed is set to be approximately 10 to 2,000 cm/sec for the irradiation.

It is to be noted that a laser using, as a medium, single crystallineYAG; YVO₄, forsterite (Mg₂SiO₄), YAlO₃, or GdVO₄, or polycrystalline(ceramic) YAG, Y₂O₃, YVO₄, YAlO₃, or GdVO₄ doped with one or more of Nd,Yb, Cr, Ti, Ho, Er, Tm, and Ta as a dopant; an Ar ion laser; or aTi:sapphire laser can be continuously oscillated. Further, pulseoscillation thereof can be performed with a repetition rate of 10 MHz ormore by carrying out Q switch operation or mode synchronization. When alaser beam is oscillated with a repetition rate of 10 MHz or more, asemiconductor film is irradiated with a next pulse while thesemiconductor film is melted by the laser beam and solidified.Therefore, unlike in a case of using a pulsed laser with a lowrepetition rate, a solid-liquid interface can be continuously moved inthe semiconductor film so that crystal grains, which continuously growtoward a scanning direction, can be obtained.

When ceramic (polycrystal) is used as a medium, the medium can be formedto have a free shape in a short time at low cost. When a single crystalis used, a columnar medium with several mm in diameter and several tensof mm in length is usually used. In the case of using the ceramic, amedium much larger can be formed.

Concentration of a dopant such as Nd or Yb in a medium, which directlycontributes to a light emission, cannot be changed largely in both casesof the single crystal and the polycrystal; therefore, there is alimitation to some extent in improvement in output of a laser byincreasing the concentration. However, in the case of the ceramic, thesize of a medium can be significantly increased as compared with thesingle crystal; therefore, drastic improvement in output of a laser canbe realized.

Further, in the case of the ceramic, a medium with a parallel hexahedronshape or a rectangular parallelepiped shape can be easily formed. In acase of using a medium having such a shape, when oscillated light ismade to travel in a zigzag inside the medium, a long path of theoscillated light can be obtained. Therefore, amplitude is increased anda laser beam can be oscillated at high output. Furthermore, across-sectional shape of a laser beam, which is emitted from a mediumhaving such a shape, is a quadrangular shape; therefore, as comparedwith a laser beam with a circular shape, the laser beam with thequadrangular shape in cross section has an advantage to be shaped into alinear beam. By shaping a laser beam emitted in the above describedmanner using an optical system, a linear beam with 1 mm or less inlength of a minor axis and several mm to several m in length of a majoraxis can be easily obtained. In addition, when a medium is uniformlyirradiated with excited light, a linear beam is emitted with uniformenergy distribution in a long side direction.

By irradiating the semiconductor film with such a linear beam, an entiresurface of the semiconductor film can be uniformly annealed. In a casewhere uniform annealing is required from one end to the other end of thelinear beam, ingenuity such as arrangement of slits on both ends of thelinear beam so as to shield an attenuated portion of energy from lightis required.

When the semiconductor film is annealed using a linear beam with uniformintensity obtained in this manner and an electronic device ismanufactured using the semiconductor film, characteristics of theelectronic device are favorable and uniform.

Subsequently, if necessary, the semiconductor film is doped with thevery small amount of impurity elements (boron or phosphorus) to controla threshold value of a TFT. Here, an ion doping method is used, in whichexcitation by plasma is conducted without conducting mass separation.

The island-like semiconductor film 3 is formed to have a thickness of 25to 80 nm (preferably, 30 to 70 nm). Although there is no limitation on amaterial for the semiconductor film, the semiconductor film ispreferably formed from silicon, a silicon-germanium (SiGe) alloy, or thelike.

Then, a gate insulating film 4 is formed to cover the island-likesemiconductor film 3. As the gate insulating film 4, a single layerstructure or a stacked structure of a thermal oxidation film, a siliconoxide film, a silicon nitride film, a silicon oxynitride film, or thelike can be used. A silicon oxide film is preferably used for the gateinsulating film 4 to be in contact with the island-like semiconductorfilm 3. This is because a trap level in an interface between the gateinsulating film 4 and the island-like semiconductor film is lowered whenthe gate insulating film 4 is formed of the silicon oxide film. Further,a silicon nitride film is preferably used for the gate insulating filmto be in contact with a gate electrode when the gate electrode is formedfrom Mo. This is because a silicon nitride film does not oxidize Mo.

Here, as the gate insulating film 4, a silicon oxynitride film(composition ratio: Si=32%, O=59%, N=7%, and H=2%) having a thickness of115 nm is formed by a plasma CVD method.

Next, a conductive layer is formed over the gate insulating film 4, anda shape of the conductive layer is processed using a mask formed by aphotolithography method or the like to form a gate electrode 5. As amaterial for the gate electrode, Mo, Ti, W, Al, Nd, Cr, an alloy ofthese elements, or the like is used. Alternatively, the gate electrode 5may be formed from these elements or formed by stacking an alloy ofthese elements. Here, the gate electrode is formed from Mo. Then, theisland-like semiconductor film 3 is doped with an impurity element usingthe gate electrode 5 or a resist as a mask to form a channel formationregion 8, and impurity regions 9 to be a source region and a drainregion.

Then, a first interlayer insulating film 6 is formed using siliconnitride. Subsequently, an impurity element with which the island-likesemiconductor film 3 is doped is activated and hydrogenated. It is to benoted that the first interlayer insulating film 6 may not be formed.

Next, a second interlayer insulating film 7 is formed using an inorganicmaterial having a light-transmitting property (silicon oxide, siliconnitride, silicon oxynitride, or the like) or an organic compoundmaterial having a low dielectric constant (a photosensitive ornonphotosensitive organic resin material). Alternatively, the secondinterlayer insulating film may be formed using a material includingsiloxane. It is to be noted that siloxane is a material composed of askeleton formed by the bond of silicon (Si) and oxygen (O), in which anorganic group containing at least hydrogen (such as an alkyl group or anaromatic hydrocarbon) is included as a substituent. Alternatively, afluoro group may be used as the substituent. Further alternatively, afluoro group and an organic group containing at least hydrogen may beused as the substituent. The second interlayer insulating film 7 mayhave a stacked structure.

Next, a mask made of a resist is formed using a photomask. The firstinterlayer insulating film 6, the second interlayer insulating film 7,and the gate insulating film 4 are selectively etched using the mask toform a contact hole. Then, the mask made of a resist is removed.

A conductive film is formed over the second interlayer insulating film 7by a sputtering method or a printing method. The conductive film may bea transparent conductive film or a film having a reflecting property. Ina case of a transparent conductive film, for example, an indium tinoxide (ITO) film in which tin oxide is mixed in indium oxide, an indiumtin silicon oxide (ITSO) film in which silicon oxide is mixed in indiumtin oxide (ITO), an indium zinc oxide (IZO) film in which zinc oxide ismixed in indium oxide, a zinc oxide film, or a tin oxide film can beused. It is to be noted that IZO is a transparent conductive materialformed by a sputtering method using a target in which 2 to 20 wt % ofzinc oxide (ZnO) is mixed in ITO.

A transparent conductive film 10 is formed over the second interlayerinsulating film 7, and subsequently, a metal film 11 is formed over thetransparent conductive film 10. The transparent conductive film 10 andthe metal film 11 can be successively formed by a sputtering method.

The metal film 11 is preferably formed from a material having resistancelower than that of the transparent conductive film because thetransparent conductive film is formed from a material having highresistance in many cases. For example, Ti, Mo, Ta, Cr, W, Al, or thelike can be used. In addition, a two-layer structure in which any of Ti,Mo, Ta, Cr, and W and Al are stacked, or a three-layer stacked structurein which Al is interposed between metal such as Ti, Mo, Ta, Cr, and Wmay be used. Then, after the entire surface of the metal film 11 iscoated with a resist film, exposure is conducted using an exposure maskshown in FIG. 1A.

In a case of using an ITO film as the transparent conductive film, astep of crystallizing the ITO film by heat treatment is required. At thetime, the metal film 11 may be formed after the ITO film is formed by asputtering method and baked. In a case of using an ITSO film, the numberof steps is decreased because a step of crystallization is not required.

In FIG. 1A, the exposure mask has light-shielding portions 12 a and 12 bby which exposure light is shielded and a semi light-transmittingportion 13 through which exposure light passes partially. The semilight-transmitting portion 13 is provided with a semi light-transmittingfilm 19 to reduce light intensity of exposure light. The light-shieldingportions 12 a and 12 b are formed by stacking a metal film 20 over thesemi light-transmitting film 19. A width of the light-shielding portion12 b is denoted by t1 and a width of the semi light-transmitting portion13 is denoted by t2. Here, an example of using a semi light-transmittingfilm for the semi light-transmitting portion is shown; however, thepresent invention is not limited thereto, and the semilight-transmitting portion is acceptable as long as light intensity ofexposure light is reduced. In addition, a diffraction grating patternmay be used for the semi light-transmitting portion.

The resist film is exposed using the exposure mask shown in FIG. 1A,thereby forming a non-exposure region 14 a and an exposure region 14 bin the resist film. In exposing the resist film, light is reflected bythe light-shielding portions 12 a and 12 b or passes through the semilight-transmitting portion 13, and accordingly, the exposure region 14 bshown in FIG. 1A is formed.

When developing is conducted, the exposure region 14 b is removed, andaccordingly, a resist pattern 15 a having two thicknesses, which areroughly divided, and a resist pattern 16 a having an almost uniformthickness are obtained over the metal film 11 as shown in FIG. 1B. Theresist pattern 15 a has a region having a thick thickness and a regionhaving a thickness thinner than the region. A thickness of the regionhaving a thin thickness can be adjusted by adjusting exposure energy ortransmittance of the semi light-transmitting film 19. The resist pattern15 a is left-right asymmetric and the resist pattern 16 a is left-rightsymmetric.

Next, the metal film 11 and the transparent conductive film 10 areetched by dry etching. The dry etching is conducted with a dry etchingapparatus using a high density plasma source such as ECR (ElectronCycrotron Resonance) and ICP (Inductive Coupled Plazma).

Here, an example of using an ICP etching apparatus is shown; however,the present invention is not limited thereto, and for example, aparallel-plate etching apparatus, a magnetron etching apparatus, an ECRetching apparatus, or a helicon-type etching apparatus may also beemployed.

Alternatively, the metal film 11 and the transparent conductive film 10may be etched by wet etching. However, dry etching is suitable formicrofabrication; therefore, dry etching is preferable. The material forthe metal film 11 and the transparent conductive film 10 is differentfrom the material for the second interlayer insulating film 7;therefore, high etching selectivity of the second interlayer insulatingfilm 7 with respect to the metal film 11 and the transparent conductivefilm 10 can be obtained even if dry etching is conducted. In order tomake etching selectivity further high, at least a top layer of thesecond interlayer insulating film 7 may be formed of a silicon nitridefilm.

In this manner, as shown in FIG. 1C, a pattern formed by stacking atransparent conductive film 17 a and a metal film 17 b, and a patternformed by stacking a transparent conductive film 18 a and a metal film18 b are formed over the second interlayer insulating film 7.

Next, ashing or etching is conducted to the resist patterns 15 a and 16a (FIG. 2A). In accordance with this step, a region having a thinthickness in the resist pattern 15 a is etched and a total thickness ofthe resist patterns 15 a and 16 a is decreased by a thickness of theregion having a thin thickness. Thus, resist patterns 15 b and 16 b areformed. The resist patterns 15 a and 16 a are etched not only in athickness direction but also a width direction; therefore, widths of theresist patterns 15 b and 16 b are decreased more than widths of themetal films 17 b and 18 b, and the transparent conductive films 17 a and18 a. Thus, sides of the resist patterns 15 b and 16 b and sides of themetal film and the transparent conductive film in the lower layer do notcoincide, and the sides of the resist patterns 15 b and 16 b recede. InFIG. 2B, the resist pattern 15 b is left-right asymmetric and the resistpattern 16 b is left-right symmetric.

Then, the metal film 18 b is etched using the resist pattern 15 b toform a metal film 18 c (FIG. 2B). The metal film 18 b is preferablyformed from a material capable of obtaining high etching selectivitywith respect to the transparent conductive film 18 a so that thetransparent conductive film 18 a is not etched unnecessarily at thistime. For example, Ti, Mo, Cr, Al, or the like is preferable as amaterial for the metal film 18 b if a material for the transparentconductive film 18 a is ITSO, and the metal film 18 b may have a stackedstructure formed from these materials. Then, the metal film 18 c with asmaller pattern, namely a smaller plane-surface area, than thetransparent conductive film 18 a is formed. On the other hand, the metalfilm 17 b is also etched using the resist pattern 16 b to form a metalfilm 17 c with a smaller plane-surface area than the transparentconductive film 17 a.

Although etching of the metal films 17 b and 18 b through FIGS. 2A to 2Bmay be conducted by dry etching or wet etching, FIG. 2B shows a casewhere the metal films 17 c and 18 c are formed by dry etching. In a caseof conducting dry etching, sides of the metal film 18 c in cross sectionare asymmetric. This is because the metal film 18 c reflecting anasymmetric shape of the resist pattern 15 b is formed. The metal film 18c has a cross-sectional shape in which one side is more inclined thanthe other side. Sides of the metal film 17 c are formed so that thesides of the metal film 17 c and sides of the resist pattern 16 bcoincide. One side of the metal film 18 c is on an extended line of oneside of the resist pattern 15 b, and the other side thereof and theother side of the resist pattern 15 b coincide.

If the metal films 17 b and 18 b are etched by wet etching, etching isconducted isotropically. Accordingly, metal films smaller than theresist patterns 15 b and 16 b are formed. FIGS. 4A and 4B show views ina case of conducting wet etching. In FIG. 4A, metal films 17 b and 18 bare etched by wet etching to form metal films 17 d and 18 d,respectively. Portions other than these in FIG. 4A are the same as inFIG. 2B.

Sides of the resist patterns 15 b and 16 b and sides of the metal films17 d and 18 d do not coincide. Therefore, the metal films 17 d and 18 d,which are further smaller, are formed by wet etching than dry etchingeven if the same resist patterns 15 b and 16 b are used as masks.

FIG. 4B is a view in a case where a metal film 17 d is formed bystacking three layers. For example, the metal film 17 d has a stackedstructure in which an aluminum film 92 a is interposed between Ti films91 a and 93 a, and sides of the metal film 17 d and a resist pattern 16b do not coincide. In addition, a metal film 18 d also has a stackedstructure in which an aluminum film 92 a is interposed between Ti films91 b and 93 b, and sides of the metal film 18 d and a resist pattern 15b do not coincide.

In FIGS. 4A and 4B, transparent conductive films 17 a and 18 a areformed by dry etching. Therefore, each side thereof has an angle θ₁,which is almost perpendicular or close to 90° with respect to asubstrate surface. On the other hand, if the metal films 17 d and 18 dare formed by wet etching, each side thereof has an acute angle θ₂ withrespect to the substrate surface by isotropic etching. Therefore, θ₁>θ₂is satisfied when the angle θ₁ of the side of the transparent conductivefilm and the angle θ₂ of the side of the metal film are compared witheach other. It is to be noted that the angle θ₁ refers to an inclinedangle of the side of the transparent conductive film with respect to thesurface of the substrate 1, and the angle θ₂ refers to an inclined angleof the side of the metal film with respect to the surface of thesubstrate 1. Each of the angles θ₁ and θ₂ is within the range of 0 to90°.

In a case where the metal film has a stacked structure as shown in FIG.4B, etching rate is different in each layer in some cases. In accordancewith this, the angles formed by the side of each layer with respect tothe substrate surface are different from each other in some cases.Therefore, in a case where the metal film is a stack, an angle formed bythe side of the film in the lowest layer with respect to the substratesurface is denoted by θ₂.

It is to be noted that the sides of the metal films 17 d and 18 d, andsides of the transparent conductive films 17 a and 18 a are not smoothbut uneven in some cases. In this case, the angles θ₁ and θ₂ may beappropriately determined. For example, the angles θ₁ and θ₂ can bedetermined using a rough straight line or curved line drawn with respectto an uneven side. Alternatively, a number of angles θ₁ and θ₂ can betaken based on an uneven side, and average values thereof can be anglesθ₁ and θ₂. The most rational method may be employed.

As described above, the metal films 17 c and 18 c or the metal films 17d and 18 d are formed by any of dry etching and wet etching. Even if themetal films are formed by either dry etching or wet etching, the metalfilms 17 c and 18 c or the metal films 17 d and 18 d having sidesreceded from the sides of the transparent conductive films 17 a and 18a, respectively, are formed. In other words, the metal film 17 c or 17 dhaving a smaller plane-surface area than the transparent conductive film17 a and the metal film 18 c or 18 d having a smaller plane-surface areathan the transparent conductive film 18 a are form led. One of thereasons is that the sizes of the resist patterns 15 a and 16 a, whichare masks for forming the transparent conductive films 17 a and 18 a,and the sizes of the resist patterns 15 b and 16 b, which are masks forforming the metal films, are different from each other, and the sizes ofthe resist patterns 15 b and 16 b are smaller than that of the resistpatterns 15 a and 16 a.

Thereafter, the resist patterns 15 b and 16 b are removed (FIG. 2C).Accordingly, a wiring or an electrode formed of the transparentconductive film 17 a and the metal film 17 c, and a wiring or anelectrode formed of the transparent conductive film 18 a and the metalfilm 18 c are formed. The transparent conductive film 18 a serves as apixel electrode. If the resist patterns 15 b and 16 b are removed fromFIGS. 4A and 4B, the wiring or the electrode formed of the transparentconductive film 17 a and the metal film 17 d, and the wiring or theelectrode formed of the transparent conductive film 18 a and the metalfilm 18 d are formed.

When the metal film 18 b is etched using the resist pattern 15 b as amask, part of the surface of the transparent conductive film 18 a isetched in some degree. In particular, when the metal film 18 c is formedby dry etching, selectivity between the transparent conductive film inthe lower layer and the metal film 18 b is difficult to obtain;therefore, part of the surface of the transparent conductive film 18 ais etched more easily. Thus, thickness a<thickness b is satisfied whenthe thicknesses a and b of the transparent conductive film 18 a in FIG.2C are compared with each other. It is to be noted that the thickness arefers to an average thickness of the transparent conductive film 18 ain a portion where the transparent conductive film 18 a does not overlapwith the metal film 18 c or the metal film 18 d, and the thickness brefers to a thickness of the transparent conductive film 18 a in abottom portion of the contact hole reaching the impurity region 9.

In a case where a light emitting element is stacked over the TFT shownin FIG. 2C to form a light emitting device which emits light in adirection of the substrate 1, a thin thickness of the transparentconductive film 18 a makes transmittance high, and accordingly, brightdisplay can be provided. Thus, the thickness a is preferably thin. Inaddition, the surface of the transparent conductive film 18 a can beetched in etching the metal film 18 b using the resist pattern 15 b as amask. Therefore, dust over the surface can be removed, and accordingly,short-circuit of the light emitting element due to dust can beprevented.

One side of the metal film 18 c formed in this embodiment mode isinclined. Accordingly, in a case where the metal film 18 c is utilizedfor a liquid crystal display device, rubbing can be conducted smoothlyon the side of the metal film 18 c when rubbing is conducted from theinclined side of the metal film 18 c. When rubbing is conducted from adirection in which the side of the metal film 18 c is perpendicular,there is a case where rubbing is conducted imperfectly for a reason suchas stress on a rubbing cloth in a portion of a perpendicular side,thereby obtaining imperfect alignment. Therefore, rubbing is preferablyconducted from the inclined side of the metal film 18 c.

As shown in FIGS. 4A and 4B, in a case where the metal films 17 d and 18d each having inclined sides are formed, rubbing can be conductedsmoothly from either direction and becomes further effective.

FIG. 3 shows a top view of FIG. 2C. FIG. 2C is a cross-sectional viewtaken along a line A-A′ in FIG. 3. As apparent from FIG. 3, the wiringor the electrode formed by stacking the transparent conductive film 17 aand the metal film 17 c serves as a source electrode or a drainelectrode of a TFT and also serves as a source wiring. In addition, thewiring or the electrode formed of the transparent conductive film 18 aand the metal film 18 c serves as a source electrode or a drainelectrode of the TFT and also serves as a pixel electrode. Strictly, aportion of the transparent conductive film 18 a which does not overlapwith the metal film 18 c serves as a pixel electrode and transmitslight. A capacitor wiring 21 is formed of the same layer as the gateelectrode 5, and the capacitor wiring 21 forms a capacitor byoverlapping with the transparent conductive film 18 a. It is to be notedthat the capacitor wiring 21 may be formed of the different layer fromthe gate electrode 5. The side of the metal film 17 c and the side ofthe transparent conductive film 17 a do not coincide, and is locatedinside the side of the transparent conductive film 17 a. The side of themetal film 18 c and the side of the transparent conductive film 18 a donot coincide, and the side of the metal film 18 c is located inside theside of the transparent conductive film 18 a. The relation between themetal film 17 d and the transparent conductive film 17 a and therelation between the metal film 18 d and the transparent conductive film18 a, which are explained in FIGS. 4A and 4B, are the same as describedabove.

In this embodiment mode, it is extremely effective to form a transparentconductive film serving as a pixel electrode over a plane surface inteems of preventing disconnection of the transparent conductive film.When the metal film 18 c is formed by etching, the surface of thetransparent conductive film 18 a in the lower layer, which is exposedfrom the metal film 18 c, is also etched in some degree. Therefore, if athickness of the transparent conductive film is nonuniform owing to theformation of the transparent conductive film 18 a over a surface havinglevel difference, a portion of the transparent conductive film having athin thickness is etched by etching for forming the metal film 18 c. Inaccordance with this, disconnection of the transparent conductive filmmay occur. When disconnection occurs, light leakage is generated in thedisconnected portion, or an area of the pixel electrode becomes smalland aperture ratio is lowered. Therefore, the portion of the transparentconductive film 18 a exposed from the metal film 18 c is preferablyformed over the plane surface. Thus, it is preferable that the secondinterlayer insulating film 7 be formed from an organic material to formthe second interlayer insulating film having a plane surface.

When a stack of the metal film and the conductive film is formed inaccordance with the present invention, a structure is obtained, in whichthe conductive film is located to be in contact with the lower portionof the metal film. However, in a portion having large level difference,the conductive film is not necessarily located to be in contact with thelower portion of the metal film. This is because there is a possibilitythat the conductive film is disconnected owing to the level difference.Therefore, favorably, the metal film is arranged over the conductivefilm in a contact hole portion reaching the impurity region 9 in FIGS.1A to 1C.

FIG. 19 shows a state where a conductive film is disconnected in acontact hole. Conductive films 94 and 95 are partially disconnectedowing to an inclined side of the contact hole. However, if the metalfilms 96 an 97 are formed over the conductive films 94 and 95 in thecontact hole portion, disconnected conductive films can be electricallyconnected to each other through the metal film even if the transparentconductive film is disconnected. In this case, there is a portion wherethe metal films 96 and 97 are in contact with the second interlayerinsulating film 7 over the side of the contact hole. The conductive filmin the contact hole portion does not serve as a pixel electrode;therefore, there is no problem remaining the metal film above.Accordingly, in the structure in this embodiment mode, electricalconnection of the transparent conductive film can be compensated by themetal film formed above even if the transparent conductive film isdisconnected in the contact hole, and accordingly, a display defect canbe prevented.

The metal film is preferably made to remain over the conductive filmalso in a portion having level difference in the conductive film due tothe capacitor wiring 21 in FIG. 3. Even if the conductive film isdisconnected due to level difference, the conductive films can beelectrically connected to each other through the metal film. Therefore,a capacitor can be certainly formed.

It is to be noted that a shape of the transparent conductive film 18 ain FIG. 3 is one example, and another shape may be used. For example, ifthe transparent conductive film 18 a has an edge with a comb shape, thetransparent conductive film 18 a can serve as a pixel electrode to beused for an IPS (In-Plane-Switching) method or an FFS (Fringe FieldSwitching) method, or a pixel electrode to be used for a MVA(Multi-domain Vertical Alignment) method or a PVA (Patterned VerticalAlignment) method by putting a slit therein.

In accordance with the above, the number of manufacturing steps can bereduced because the transparent conductive film and the metal film canbe formed using one resist pattern. In addition, by stacking the metalfilm, resistance can be lowered and conductivity can be enhanced whilethe transparent conductive film is utilized as the wiring or theelectrode.

In a case where the resist patterns 15 a and 16 a are naturally etchedto be the resist patterns 15 b and 16 b during etching of thetransparent conductive film 10 and the metal film 11 from a state shownin FIG. 1B, a step of forming the resist patterns 15 b and 16 b byashing or etching the resist pattern may not be provided.

This embodiment mode is explained using a top-gate TFT having anisland-like semiconductor film formed of a crystalline semiconductorfilm. However, this embodiment mode can also be applied to a bottom-gateTFT formed of a crystalline semiconductor film. In addition, in thisembodiment mode, the island-like semiconductor film has the impurityregions 9 to be a source region and a drain region and the channelformation region 8; however, a low concentration impurity region, anoffset region, and the like can be provided in addition to these.

Embodiment Mode 2

This embodiment mode will be explained with reference to FIGS. 5A to 5C.As for a forming method, a material, and the like of a type of asubstrate and each layer which form a TFT to be explained in thisembodiment mode, see Embodiment Mode 1.

An insulating film 402 is formed as a base film over a substrate 401. Itis to be noted that a base film may not be provided. Then, a conductivelayer is formed over the insulating film 402, and a shape of theconductive layer is processed using a mask formed by a photolithographymethod or the like to form a gate electrode 403.

A gate insulating film 404 is formed to cover the gate electrode 403. Anamorphous semiconductor film is formed over the gate insulating film404. Although there is no limitation on a material for the amorphoussemiconductor film, the amorphous semiconductor film is favorably formedfrom silicon, a silicon-germanium (SiGe) alloy, or the like.Subsequently, a conductive layer is formed over the amorphoussemiconductor film. The conductive layer can be formed of, for example,an amorphous silicon film including phosphorus. Then, shapes of theamorphous semiconductor film and the conductive layer are processedusing a mask formed by a photolithography method or the like to form anisland-like semiconductor film 405 and a conductive layer 406.

A transparent conductive film 407 and a metal film 408 are stacked overthe conductive layer 406. It is to be noted that a conductive layerhaving a reflecting property may be used instead of the transparentconductive film. As the transparent conductive film, the material forthe transparent conductive film shown in Embodiment Mode 1 can be used.Then, after the entire surface of the metal film 408 is coated with aresist film, exposure is conducted using an exposure mask shown in FIG.5A.

In FIG. 5A, the exposure mask has light-shielding portions 409 a and 409b and a semi light-transmitting portion 410. A diffraction pattern or asemi light-transmitting film can be used for the semi light-transmittingportion 410. When the resist film is exposed using the exposure maskshown in FIG. 5A, a non-exposure region 411 and an exposure region 412are formed in the resist film. Then, developing is conducted to formresist patterns 413 a and 414 a as shown in FIG. 5B. The resist pattern414 a includes a developed region 422 (a portion of the resist pattern414 a on the left side of a dashed line), which corresponds to thelight-shielding portion 409 b during the exposure, and a developedregion 423 (a portion of the resist pattern 414 a on the right side ofthe dashed line), which corresponds to the semi light-transmittingportion 410 during the exposure.

Next, the metal film 408 and the transparent conductive film 407 areetched by dry etching. Accordingly, as shown in FIG. 5C, a patternformed by stacking a transparent conductive film 415 and a metal film416 and a pattern formed by stacking a transparent conductive film 419and a metal film 420 are formed. The etching may be conducted by wetetching. However, dry etching is suitable for microfabrication;therefore, dry etching is preferable. The materials for the metal film408 and the transparent conductive film 407 are different from thematerial for the gate insulating film 404; therefore, high etchingselectivity can be obtained even if dry etching is conducted. Further,in order to make the etching selectivity high, at least a top layer ofthe gate insulating film 404 may be formed of a silicon nitride film.

Next, as shown in FIG. 6A, ashing or etching is conducted to the resistpatterns 413 a and 414 a. In accordance with this step, the region 423of the resist pattern 414 a is removed, and a thickness of the region422 of the resist pattern 414 a becomes thinner by a thickness d2 ofthis region 423 to form a resist pattern 414 b. Ashing of the resistpattern 413 a is also conducted by the thickness d2 to form a resistpattern 413 b. Further, etching is conducted also in a width direction;therefore, widths of the resist patterns 413 b and 414 b become smallerthan widths of a metal film 416, the metal film 420, and the transparentconductive films 415 and 419. Therefore, sides of the resist patterns413 b and 414 b and sides of the metal films and the transparentconductive films in the lower layer do not coincide, and the sides ofthe resist patterns 413 b and 414 b recede. Further, angles of the sidesof the resist pattern 414 b with respect to a substrate surface aredifferent from each other. On the other hand, angles of the sides of theresist pattern 413 b with respect to the substrate surface are almostthe same.

Then, the metal film 416 is etched using the resist pattern 414 b toform a metal film 421. Further, the metal film 420 is etched using theresist pattern 413 b to form a metal film 424 (FIG. 6B). At this time,the transparent conductive film 415 is not etched unnecessarily. Themetal films 424 and 421 are formed to have patterns smaller than thetransparent conductive films 419 and 415. The conductive layer 406 isetched using the transparent conductive films 419 and 415 as masks toform conductive layers 417 and 418. Part of the island-likesemiconductor film 405 is etched somewhat. One edge portion of thetransparent conductive film 419 and one edge portion of the conductivelayer 417 coincide, and one edge portion of the transparent conductivefilm 415 and one edge portion of the conductive layer 418 coincide. Themetal films 421 and 424 are formed in the same step.

In addition, the conductive layer 406 may be etched at the same time asetching for forming the metal films 421 and 424.

Then, the resist patterns 413 b and 414 b are removed to form a wiringor an electrode formed of the transparent conductive film 419 and themetal film 424, and a wiring or an electrode formed of the metal film421 and the transparent conductive film 415. The transparent conductivefilm 415 serves as a pixel electrode (FIG. 6C).

Although the conductive layers 417 and 418 can be formed at the sametime as etching of FIG. 5C, it is preferable to etch the conductivelayers 417 and 418 in forming or after forming the metal films 424 and421 as shown in FIGS. 6A to 6C. This is because there is a possibilitythat the island-like semiconductor film is further etched in faultingthe metal films 424 and 421 if the island-like semiconductor film isexposed in a stage of FIG. 5C.

Etching of FIG. 6B may be conducted by dry etching or wet etching. In acase of conducting dry etching, a cross-sectional shape of the metalfilm 421 reflects a shape of the resist pattern 414 b to be left-rightasymmetric as shown in FIGS. 6B and 6C. In other words, the metal film421 has a cross-sectional shape in which one side is more inclined thanthe other side. One side of the metal film 421 and one side of theresist pattern 414 b coincide, and the other side of the metal film 421is on an extended line of the other side of resist pattern 414 b. Themetal film 424 is formed so that a side thereof and a side of the resistpattern 413 b coincide.

A case of forming the metal films 421 and 424 by wet etching will beexplained with reference to FIG. 8. The metal films 421 and 424 formedby dry etching are substituted by metal films 425 and 426 in a case ofbeing farmed by wet etching.

In a case of wet etching, as shown in FIG. 8, the metal films 425 and426 smaller than resist patterns 413 b and 414 b are formed, and sidesof the resist patterns 413 b and 414 b and sides of the metal films 425and 426 do not coincide. Therefore, the metal film having a furthersmaller plane-surface area is formed by wet etching than dry etchingeven if the same resist patterns 413 b and 414 b are used as masks. Inthe same manner as FIGS. 4A and 4B, in a case where the metal film isformed by wet etching, θ₁>θ₂ is satisfied when an angle θ₁ of a side oftransparent conductive films 415 and 419 and an angle θ₂ of the side ofthe metal films 425 and 426 are compared with each other. It is to benoted that the angle θ₁ refers to an inclined angle of the side of thetransparent conductive film with respect to the surface of the substrate401, and the angle θ₂ refers to an inclined angle of the side of themetal film with respect to the surface of the substrate 401. Each of theangles θ₁ and θ₂ is within the range of 0 to 90°. Alternatively, whenthe metal films 425 and 426 each have a stacked structure as shown inFIG. 4B, an angle of a side of a film in the lowest layer with respectto the substrate surface is denoted by θ₂.

It is to be noted that, in a case of wet etching, the conductive layer406 may be etched at the same time as etching in FIG. 5C or may beetched after forming the metal films 425 and 426 in FIG. 6B.

Even if the metal films are formed by either dry etching or wet etching,the metal film 425 or 424 having sides receding from the sides of thetransparent conductive film 419, and the metal film 421 or 426 havingsides receding from the sides of the transparent conductive film 415 areformed. In other words, the metal film 424 or 425 having a smallerplane-surface area than the transparent conductive film 419 and themetal film 421 or 426 having a smaller plane-surface area than thetransparent conductive film 415 are formed.

Then, the resist patterns 413 b and 414 b are removed, and a wiring oran electrode formed of the transparent conductive film 419 and the metalfilm 424, and a wiring or an electrode formed of the metal film 421 andthe transparent conductive film 415 are formed (FIG. 6C). If the resistpatterns 413 b and 414 b are removed from FIG. 8, the wiring or theelectrode formed of the transparent conductive film 419 and the metalfilm 425, and the wiring or the electrode formed of the transparentconductive film 415 and the metal film 426 are formed.

If a stack of the metal film 421 and the transparent conductive film 415is formed using the resist pattern 414 a having regions different inthickness of the present invention, part of the surface of thetransparent conductive film 415 is etched in some degree in forming themetal film 421. In particular, when the metal film 421 is formed by dryetching, selectivity between the transparent conductive film 415 in thelower layer and the metal film 421 is difficult to obtain; therefore,part of the surface of the transparent conductive film 415 is etchedmore easily. Thus, thickness a<thickness c is satisfied when thethickness a (a thickness of the transparent conductive film 415 exposedfrom the metal film 421) and the thickness c (a thickness of thetransparent conductive film to be in contact with the gate insulatingfilm 404 and the metal film 421) of the transparent conductive film 415in FIG. 6C are compared with each other. It is to be noted that each ofthe thicknesses a and c is an average thickness.

In a case where a light emitting element is stacked over the TFT shownin FIG. 6C to form a light emitting device, the following effect can beobtained by thickness a<thickness c. If a light emitting device whichemits light in a direction of the substrate 401 is used, a thinthickness a can provide bright display. In addition, the surface of thetransparent conductive film 415 can be etched; therefore, dust over thesurface can be removed, and accordingly, short-circuit of the lightemitting element can be prevented.

One side of the metal film 421 formed in this embodiment mode isinclined. Accordingly, in a case where the metal film 421 is utilizedfor a liquid crystal display device, rubbing can be conducted smoothlyon the side of the metal film 421 when rubbing is conducted from theinclined side of the metal film 421. When rubbing is conducted from adirection in which the side of the metal film 421 is perpendicular,there is a case where rubbing is conducted imperfectly for a reason suchas stress on a rubbing cloth in a portion of a perpendicular side,thereby obtaining imperfect alignment. Therefore, rubbing is preferablyconducted from the inclined side of the metal film 421.

As shown in FIG. 8, in a case where the metal films 425 and 426 eachhaving inclined sides are formed by wet etching, rubbing can beconducted smoothly from either direction and becomes further effective.

FIG. 7 is a top view of FIG. 6C. FIG. 6C is a cross-sectional view takenalong a line A-A′ in FIG. 7. As shown in FIG. 7, the wiring or theelectrode formed by stacking the transparent conductive film 419 and themetal film 424 serves as a source electrode or a drain electrode of aTFT and also serves as a source wiring. In addition, the wiring or theelectrode formed of the transparent conductive film 415 and the metalfilm 421 serves as a source electrode or a drain electrode of the TFTand also serves as a pixel electrode. Strictly, a portion of thetransparent conductive film 415 which does not overlap with the metalfilm 421 serves as a pixel electrode. A capacitor wiring 430 is formedof the same layer as the gate electrode 403, and the capacitor wiring430 forms a capacitor by overlapping with the transparent conductivefilm 415. It is to be noted that the capacitor wiring 430 may be formedof the different layer from the gate electrode. The side of the metalfilm 424 and the side of the transparent conductive film 419 do notcoincide, and the side of the metal film 424 is located inside the sideof the transparent conductive film 419. The side of the metal film 421and the side of the transparent conductive film 415 do not coincide, andthe side of the metal film 421 is located inside the side of thetransparent conductive film 415. The relation between the metal film 425and the transparent conductive film 419 and the relation between themetal film 426 and the transparent conductive film 415, which areexplained in FIG. 8, are the same as described above.

In addition, it is effective to cover the transparent conductive film415, which is formed over level difference due to the capacitor wiring430, the gate electrode 403, or the island-like semiconductor film 405,with the metal film 421 in terms of preventing disconnection of thetransparent conductive film serving as a pixel electrode. When the metalfilm 421 is formed by etching, the transparent conductive film 415 isalso etched in some degree. Therefore, if a thickness of the transparentconductive film is nonuniform, disconnection of the transparentconductive film occurs in this etching. Therefore, it is preferable toutilize a portion of the transparent conductive film over a planesurface over which a uniform thickness is easily formed, as a pixelelectrode. Thus, the metal film 421 may be formed to cover thetransparent conductive film 415 located over the surface having leveldifference. Accordingly, the transparent conductive film 415 over thesurface having level difference is not etched nor disconnected.

In order to cover the transparent conductive film 415 over the surfacehaving level difference with the metal film 421, it is necessary tosatisfy at least d1>d2 when, in FIG. 5B, a thickness of the region 423of the resist pattern 414 a is denoted by d2 and a thinnest thickness inthe region 422 is denoted by d1. The reason is as follows: Although athickness of the entire resist is thinned by d2 by ashing of thethickness d2 in conducting ashing to the resist in FIG. 6A, it isnecessary that a resist remains in the region 422 even if the thicknessis thinned by d2 by the ashing. Therefore, it is preferable that thethickness d1 of a portion having a thinnest thickness of the region 422be thicker than the thickness d2 of the region 423 at least in theresist pattern 414 a.

In accordance with the above steps, a bottom-gate TFT having anisland-like semiconductor film formed of an amorphous semiconductor filmcan be formed. By stacking the metal film, resistance can be lowered andconductivity can be enhanced while the transparent conductive film isutilized as the wiring or the electrode. Further, the number of stepscan be reduced because a resist pattern for forming the metal film 421is not required to be specially provided.

FIG. 9A shows a TFT structure having a channel protective film asanother TFT structure of this embodiment mode. In the TFT in FIG. 9A,the same portions as in FIGS. 5A to 5C, FIGS. 6A to 6C, and FIGS. 7 and8 are denoted by the same reference numerals and detailed explanationwill be omitted.

Steps up to the formation of an island-like semiconductor film 405 overa substrate 401 are the same as in FIG. 5A. Next, an insulating filmsuch as a silicon nitride film is formed and a shape of the insulatingfilm is processed by etching to form a channel protective film 601 nearthe center over the island-like semiconductor film 405. Thereafter, aconductive layer 406, a transparent conductive film 407, and a metalfilm 408 are sequentially formed so as to cover the channel protectivefilm 601. The entire surface of the metal film 408 is coated with aresist film. After exposing the resist film using an exposure maskhaving a semi light-transmitting portion, developing is conducted toform resist patterns 413 a and 414 a.

Next, etching is conducted by dry etching using the resist patterns 413a and 414 a to form conductive layers 417 and 418, transparentconductive films 415 and 419, and metal films 416 and 420. Thetransparent conductive film 415 serves as a pixel electrode (FIG. 9B).The channel protective film 601 is to be a protective film forpreventing the island-like semiconductor film 405 from being etched informing the conductive layers 417 and 418.

Next, ashing of the resist patterns 413 a and 414 a is conducted to formresist patterns 413 b and 414 b (FIG. 9C). The metal films 420 and 416are etched using the resist patterns 413 b and 414 b to form metal films424 and 421 (FIG. 9D). FIG. 9D shows a case where the metal films 424and 421 are formed by dry etching. It is to be noted that the metalfilms 425 and 426 shown in FIG. 8 may be formed by wet etching. Shapesof edge portions of the metal film and the transparent conductive filmat the time are similar to that explained in FIG. 8.

The TFT structure having the channel protective film 601 has thefollowing effect. First, there is no concern that the island-likesemiconductor film is etched in conducting dry etching in an etchingstep of the transparent conductive film 407 and the metal film 408 shownin FIG. 9B. Therefore, freedom degree of the etching step of thetransparent conductive film and the metal film is enhanced and etchingcan be conducted under an ideal etching condition. Further,microfabrication can be conducted by dry etching. Furthermore, theisland-like semiconductor film 405 can be formed to be thin and TFTcharacteristics can be improved. Therefore, the TFT structure is idealfor an active matrix organic light emitting diode which requires a TFTfeeding a large amount of current to a driving TFT.

FIG. 10 shows another TFT structure. The structure is a bottom-gate TFTformed using a crystalline semiconductor film. Steps up to the formationof a gate insulating film 404 over a substrate 401 are the same as FIG.5A. Then, a crystalline semiconductor film is formed over the gateinsulating film. The crystalline semiconductor film may be directlyformed over the gate insulating film, or the crystalline semiconductorfilm may be formed by forming and then crystallizing an amorphoussemiconductor film as in Embodiment Mode 1. A shape of the crystallinesemiconductor film is processed by etching to form an island-likesemiconductor film 405. The island-like semiconductor film 405 isselectively doped with an impurity to form a pair of impurity regions602 and a channel formation region 603 in the island-like semiconductorfilm 405. After forming an interlayer insulating film 604 over theisland-like semiconductor film 405, a contact hole reaching the impurityregion 602 is formed in the interlayer insulating layer 604, and atransparent conductive film and a metal film are stacked. The metal filmis stacked over the transparent conductive film. Then, etching isconducted using the resist pattern, which is exposed using the exposuremask shown in FIG. 5A and developed, to form an electrode or a wiringformed of the transparent conductive film 419 and the metal film 424,and an electrode or a wiring formed of the metal film 421 and thetransparent conductive film 415. If the interlayer insulating film 604is formed from an organic resin material or the like in the structure ofFIG. 10, the interlayer insulating film 604 has a plane surface. Inother words, the transparent conductive film 415 can be formed over theplane surface, and accordingly, disconnection of the transparentconductive film 415 in etching for forming the metal film 421 can beprevented.

It is to be noted that the TFT shown in FIG. 10 may have an impurityregion other than the pair of impurity regions 602.

In FIGS. 9A to 9D and FIG. 10, a feature of a shape of the metal filmdue to an etching method for forming the metal films 421 and 424 is thesame as that described above. Instead of the metal films 421 and 424,the metal films 425 and 426 each having a shape as in FIG. 8 can beformed by wet etching, or a metal film having a stacked structure may beused. Although the transparent conductive film is used as a conductivefilm serving as a pixel electrode, a reflective type conductive film maybe used. As a material for the transparent conductive film, the materialshown in Embodiment Mode 1 can be used.

This embodiment mode can be freely combined with Embodiment Mode 1within the range of enablement.

Embodiment Mode 3

In this embodiment mode, an exposure mask used in Embodiment Modes 1 and2 will be explained with reference to FIGS. 11A to 11D. FIGS. 11A to 11Ceach show a top view of the light-shielding portion 12 b and the semilight-transmitting portion 13 of the exposure mask shown in FIG. 1A orFIG. 5A. A width of the light-shielding portion 12 b of the exposuremask is denoted by t1, and a width of the semi light-transmittingportion 13 thereof is denoted by t2.

The semi light-transmitting portion 13 can be provided with adiffraction grating pattern, and FIGS. 11A and 11B each show adiffraction grating pattern having a slit portion including a pluralityof slits at or below a resolution limit of an exposure apparatus. Thediffraction grating pattern is a pattern in which at least one patternsuch as a slit or a dot is arranged. In a case where a plurality ofpatterns such as a slit or a dot is arranged, the patterns may bearranged periodically or aperiodically. By using a minute pattern at orbelow a resolution limit, the substantial amount of exposure can bemodulated and a thickness of an exposed resist after development can beadjusted.

The slit of the slit portion may be extended in a direction parallel toone side of a light-shielding portion 303 like a slit portion 301 or ina direction perpendicular to one side of the light-shielding portion 303like a slit portion 302. Alternatively, the slit of the slit portion maybe extended in an oblique direction with respect to one side of thelight-shielding portion 303. It is to be noted that a resist to be usedin this photolithography step is preferably a positive type resist.

In addition, as another example of the semi light-transmitting portion,FIG. 11C shows an example of providing a semi light-transmitting film304 having a function of reducing light intensity of exposure light. Asthe semi light-transmitting film, MoSi, MoSiO, MoSiON, CrSi, or the likecan be used as well as MoSiN. An exposure method using an exposure maskprovided with a semi light-transmitting portion is also referred to as ahalf-tone exposure method.

When the exposure masks shown in FIGS. 11A to 11C are irradiated withexposure light, the light intensity is approximately zero in thelight-shielding portion 303 and the light intensity is approximately100% in a light-transmitting portion 305. On the other hand, theintensity of light passing through the semi light-transmitting portionhaving a light intensity reduction function formed of the slit portion301 or 302 or the semi light-transmitting film 304, can be adjusted inthe range of 10 to 70%. FIG. 11D shows a typical example of the lightintensity distribution. In a case where the semi light-transmittingportion is a diffraction grating pattern, adjustment of the intensity oflight passing through the semi light-transmitting portion can berealized by adjustment of the pitch and the slit width of the slitportions 301 and 302.

This embodiment mode can be freely combined with Embodiment Modes 1 and2.

Embodiment Mode 4

In this embodiment mode, an EL (Electro Luminescence) display devicewill be explained with reference to FIGS. 12A and 12B and FIGS. 13A and13B. As for a forming method, a material, and the like of a substrateand each layer which form a TFT, see Embodiment Modes 1 and 2. Althougha TFT in FIGS. 12A and 12B and FIGS. 13A and 13B will be explained usinga top-gate TFT structure of Embodiment Mode 1, a bottom-gate TFTstructure may be used. The same portions as those in FIGS. 1A to 1C andFIGS. 2A to 2C in Embodiment Mode 1 are denoted by the same referencenumerals and detailed explanation will be omitted. However, a pixelstructure is not limited to ones in FIGS. 12A and 12B and FIGS. 13A and13B, and another pixel structure may be used.

FIG. 12A shows a top view of a pixel portion of an EL display device. Apixel is provided with two TFTs of a switching TFT 140 and a driving TFT141 for controlling current fed to an EL element. A gate electrode 5 bof the driving TFT 141 is electrically connected to a transparentconductive film 123 and a metal film 124 to be a source electrode or adrain electrode of the switching TFT 140. FIG. 12B is a cross-sectionalview taken along lines A-A′ and B-B′ in FIG. 12A.

The TFTs are formed over a substrate 1 by a method of Embodiment Mode 1.An insulating film 2 is formed over the substrate 1, and island-likesemiconductor films 3 a and 3 b are formed thereover. The island-likesemiconductor films 3 a and 3 b are formed of an amorphous semiconductorfilm or a crystalline semiconductor film. Subsequently, a gateinsulating film 4 and gate electrodes 5 a and 5 b are formed. The gateelectrode 5 a is extended from a gate wiring, and the gate electrode 5 bis formed to be separated from the gate wiring (gate electrode 5 a). Theisland-like semiconductor films 3 a and 3 b are doped with an impurityelement using the gate electrodes 5 a and 5 b as masks to form a pair ofimpurity regions and a channel formation region in the respectiveisland-like semiconductor films 3 a and 3 b. Then, a first interlayerinsulating film 6 and a second interlayer insulating film 7 are formedover the gate electrodes 5 a and 5 b.

Next, the gate insulating film 4, the first interlayer insulating film6, and the second interlayer insulating film 7 are etched to form acontact hole reaching the pair of impurity regions in the island-likesemiconductor film. At the same time, the first interlayer insulatingfilm 6 and the second interlayer insulating film 7 are etched to form acontact hole reaching the gate electrode 5 b. A transparent conductivefilm is formed over the second interlayer insulating film 7, and a metalfilm is stacked thereover. Then, the transparent conductive film and themetal film are etched by the same method as Embodiment Mode 1 to form awiring or an electrode formed of a metal film 122 and a transparentconductive film 121, a wiring or an electrode formed of the metal film124 and the transparent conductive film 123, a wiring or an electrodeformed of a metal film 126 and a transparent conductive film 125, and awiring or an electrode formed of a metal film 128 and a transparentconductive film 127. The transparent conductive film 127 serves as apixel electrode.

The metal films 122, 124, and 126 are almost similar to the transparentconductive films 121, 123, and 125 located in the lower layer,respectively, and each have a pattern with a size smaller than therespective transparent conductive films. The metal films 122, 124 and126 are formed by etching using a resist pattern which is exposed usingan exposure mask having a light-shielding portion and developed like theresist pattern 16 a shown in FIGS. 1A to 1C. On the other hand, part ofthe transparent conductive film 127 serves as a pixel electrode;therefore, the metal film 128 is not necessarily similar to thetransparent conductive film 127 and has a smaller pattern than thetransparent conductive film 127. Therefore, in order to form thetransparent conductive film 127 and the metal film 128, etching isconducted using a resist pattern which is exposed using an exposure maskhaving a semi light-transmitting portion and a light-shielding portionand developed like the resist pattern 15 a shown in FIGS. 1A to 1C.

After forming the metal films 122, 124, 126, and 128, residue of themetal film over the transparent conductive film may be removed bypolishing the surface of a portion of the transparent conductive film127 which is exposed from the metal film 128. The polishing can beconducted by a CMP (Chemical-Mechanical Polishing) method or the like.Since an electroluminescent layer, which is to be subsequently formedover the transparent conductive film 127, is extremely thin, theelectroluminescent layer is not formed to be uniform due to residue ofthe metal film, and accordingly, short-circuit between the transparentconductive film 127 and a conductive layer 131 over theelectroluminescent layer occurs. The polishing is effective inpreventing this short-circuit.

Thereafter, an insulating film 129 to be an embankment (also referred toas a partition wall) is formed over the TFT. The insulating film 129 isformed so that a portion of the transparent conductive film 127 servingas the pixel electrode is exposed. Further, the insulating film 129 isformed to cover the metal film 128. This is for preventing cause ofshort-circuit failure of an EL element due to exposure of the metal film128 from the insulating film 129. On the other hand, the insulating film129 is formed to be continuously decreased in thickness and to have acurved surface near a portion where the transparent conductive film 127is exposed. This is for preventing disconnection of theelectroluminescent layer to be formed above in level difference of theinsulating film 129. With such a shape of the insulating film 129 havinga curved surface, there is a concern that an edge portion of the metalfilm 128 is easily exposed from the insulating film 129. However, themetal film 128 formed in accordance with the present invention has astructure not to be easily exposed from the insulating film 129 becausean edge portion thereof is inclined or has an angle θ₂ as explained inEmbodiment Modes 1 and 2; therefore, it is very suitable for an ELdisplay device.

In a case where the metal film 128 is formed by dry etching, a shape ofa resist pattern to be formed above is reflected. In a cross-sectionalview of FIG. 12B, the edge portion of the metal film 128 near the curvedsurface of the insulating film 129 is more inclined than the other edgeportion. In a top view of FIG. 12A, two sides, which are farther fromthe edge portion of the transparent conductive film 127, are moreinclined than the other two sides among the four sides of the metal film128. On the other hand, in a case where the metal film 128 is formed bywet etching, the edge portion of the metal film 128 has an angle θ₂,which is more acute than an angle θ₁ in the edge portion of thetransparent conductive film 127. Accordingly, the edge portion of themetal film 128 near the curved surface of the insulating film 129 isformed to have an incline or the angle θ₂ by either forming method.Therefore, the metal film 128 has a shape which is not easily exposedfrom the insulating film 129.

Subsequently, an electroluminescent layer 130 is formed to be in contactwith the transparent conductive film 127 exposed from the insulatingfilm 129, and then, a conductive layer 131 is formed. In the abovestructure, if the TFT for driving a light emitting element is ann-channel TFT, the transparent conductive film 127 corresponds to acathode, and the conductive layer 131 corresponds to an anode. When atransparent conductive film is used for the conductive layer 131, adisplay device which emits light both above and below is obtained.

FIGS. 13A and 13B show an EL display device having a different structurefrom that of FIGS. 12A and 12B. FIG. 13A is a top view of a pixelportion of the EL display device, and FIG. 13B is a cross-sectional viewtaken along lines A-A′ and B-B′ in FIG. 13A.

Steps up to the formation of a contact hole reaching a pair of impurityregions of an island-like semiconductor film after forming a secondinterlayer insulating film 7 and conducting etching in FIG. 13B are thesame as in FIG. 12B. A switching TFT 1101 and a driving TFT 1102 areformed. Then, a conductive layer is formed and etched to form wirings orelectrodes 1103 a to 1103 d.

A third interlayer insulating film 1104 is formed over the wirings orelectrodes 1103 a to 1103 d. The third interlayer insulating film 1104is preferably formed of an organic resin film. In this manner, atransparent conductive film which is formed over the third interlayerinsulating film 1104 and serves as a pixel electrode can be formed overa plane surface.

The third interlayer insulating film 1104 is etched to form a contacthole reaching the wiring or electrode 1103 d. A transparent conductivefilm and a metal film are stacked over the third interlayer insulatingfilm 1104 and etched to form a transparent conductive film 1105 and ametal film 1106. The transparent conductive film 1105 and the metal film1106 are etched using a resist pattern which is exposed using anexposure mask having a semi light-transmitting portion and developedlike the resist pattern 15 a shown in FIGS. 1A to 1C. The transparentconductive film 1105 serves as the pixel electrode.

The contact hole formed in the second interlayer insulating film 7 andthe contact hole formed in the third interlayer insulating film 1104 arefavorably formed to overlap. The overlapping of the contact holesenables aperture ratio to be high. On the other hand, although a problemon disconnection of the transparent conductive film 1105 is caused byenlargement of level difference in the contact hole, the problem ondisconnection can be compensated by making the metal film 1106 remainover the transparent conductive film 1105 in a contact hole portion.

After forming the metal film 1106, an insulating film 129, anelectroluminescent layer 130, and a conductive layer 131 are formed inthe same manner as FIG. 12B.

In this embodiment mode, although the conductive film to be used as apixel electrode is formed of the transparent conductive film, areflective type conductive film may also be used. As a material for thetransparent conductive film, the material shown in Embodiment Mode 1 canbe used. In addition, this embodiment mode can be freely combined withEmbodiment Modes 1 to 3 within the range of enablement.

Embodiment Mode 5

In this embodiment mode, an example of a case where the presentinvention is applied to a liquid crystal display device will beexplained.

First, a manufacturing method of a liquid crystal display panel will beexplained with reference to FIG. 14A. A bottom-gate TFT is formed over asubstrate 401 in the same manner as FIG. 6C of Embodiment Mode 2. A TFTstructure is not limited to the TFT shown in FIG. 6C of Embodiment Mode2, and various kinds of TFT structures can be used.

After forming the TFT by the method of Embodiment Mode 3, an alignmentfilm 801 is formed so as to cover metal films 424 and 421 andtransparent conductive films 419 and 415. Then, a substrate 805 overwhich a color filter 802, a light-shielding film 807, an oppositeelectrode 803, and an alignment film 804 are formed is prepared, and thesubstrate 401 and the substrate 805 are attached to each other with asealing material (not shown). The light-shielding film 807 is arrangedso as to overlap with the TFT, and the color filter 802 is arranged soas to overlap with a portion of the transparent conductive film 415serving as a pixel electrode. Thereafter, a liquid crystal 806 isinjected. Thus, a display device provided with a display function iscompleted. Although not shown, a polarizing plate is attached to thesubstrates 401 and 805 on the opposite side of the liquid crystal 806.In accordance with the above steps, a liquid crystal display panel iscompleted. It is to be noted that a reflective type conductive film canbe used in substitution for the transparent conductive film.

Next, in this embodiment mode, arrangement of the metal film formed overthe transparent conductive film in a liquid crystal display device willbe explained. FIG. 14B shows one example of a top view of a liquidcrystal display device, and FIG. 14A is a cross-sectional view takenalong a line A-A′ of FIG. 14B. An island-like semiconductor film 405overlaps a gate wiring 403, and a gate wiring portion which overlapswith the island-like semiconductor film 405 is to be a gate electrode.In other words, reference numeral 403 denotes a gate wiring and a gateelectrode. Further, a stacked film of the metal film 424 and thetransparent conductive film 419 to be a source wiring is electricallyconnected to the island-like semiconductor film 405 through a conductivelayer 417, and a stacked film of the metal film 421 and the transparentconductive film 415 to be a drain wiring is electrically connected tothe island-like semiconductor film 405 through a conductive layer 418. Acapacitor wiring 808 forms a capacitor in a portion where thetransparent conductive film 415 and the capacitor wiring 808 overlap.The capacitor wiring 808 may be formed of the same layer as the gatewiring 403 or may be formed of another layer. The light-shielding film807 is indicated by a dashed line. The light-shielding film 807 overlapswith the source wiring, the drain wiring, and the TFT; however, thelight-shielding film 807 does not overlap with a portion serving as apixel electrode of the transparent conductive film 415.

The metal film 421 over the transparent conductive film 415 is formedalong an edge of the transparent conductive film 415. For details, aside of the metal film 421 is formed along a side of the transparentconductive film 415. However, the side of the metal film 421 and theside of the transparent conductive film 415 do not coincide, and theside of the metal film 421 is located inside the side of the transparentconductive film 415. By forming the metal film 421 along the edge of thetransparent conductive film 415 in this manner, arrangement accuracy ofthe light-shielding film 807 for light shielding between pixelelectrodes can be lowered. This is because the metal film 421 serves asa light-shielding film even if misalignment of the light-shielding film807 is caused in some degree, and accordingly, misalignment of thelight-shielding film 807 is acceptable within the range of existence ofthe metal film 421. In particular, in a case where the light-shieldingfilm is provided for the opposite substrate like FIGS. 14A and 14B, highalignment accuracy is desired. Therefore, it is effective to form themetal film 421 over the transparent conductive film along an edge of thepixel electrode to certainly conduct light shielding between pixels.

FIGS. 15A and 15B will be explained as another structure of a liquidcrystal display device. FIGS. 15A and 15B show examples in which aninterlayer insulating film is provided for the structure of FIGS. 14Aand 14B. FIG. 15A is a cross-sectional view taken along a line A-A′ of atop view of a liquid crystal display device shown in FIG. 15B. Anisland-like semiconductor film 405 overlaps a gate wiring 403, and agate wiring portion which overlaps with the island-like semiconductorfilm 405 is to be a gate electrode. Further, a source wiring 501 iselectrically connected to the island-like semiconductor film 405 througha conductive layer 417, and a drain wiring 502 is electrically connectedto the island-like semiconductor film 405 through a conductive layer418. A capacitor wiring 808 forms a capacitor in a portion where thetransparent conductive film 504 and the capacitor wiring 808 overlap.The capacitor wiring 808 may be formed of the same layer as the gatewiring 403 or may be formed of another layer.

An interlayer insulating film 503 is formed over the source wiring 501and the drain wiring 502, and a contact hole reaching the drain wiring502 is formed in the interlayer insulating film 503. The interlayerinsulating film 503 is an organic resin film or an inorganic insulatingfilm. The transparent conductive film 504 and a metal film 505 areformed over the interlayer insulating film 503. In a case where theinterlayer insulating film 503 is formed of an organic resin film, leveldifference due to the gate electrode 403 or the island-likesemiconductor film 405 can be reduced; therefore, the transparentconductive film 504 serving as a pixel electrode can be formed over aplane surface. Accordingly, a pixel electrode can be enlarged more thanthe structure of FIGS. 14A and 14B, and aperture ratio can be improved.

The transparent conductive film 504 and the metal film 505 are formed byetching using a resist pattern which is exposed using an exposure maskhaving a light-shielding portion and developed like the resist pattern16 a shown in FIGS. 1A to 1C. A connection portion of the transparentconductive film 504 and the drain wiring 502 has large level difference,and there is a possibility that the transparent conductive film 504 isdisconnected. Therefore, it is favorable that the metal film 505 be madeto remain over the transparent conductive film 504.

Also in the top view of FIG. 15B, the metal film 505 is formed along anedge of the transparent conductive film 504 in the same manner as FIG.14B, and can serve as part of a light-shielding film.

As the transparent conductive film 504, the material for the transparentconductive film shown in Embodiment Mode 1 can be used.

FIGS. 16A and 16B show an example of utilizing a metal film over atransparent conductive film for aligning liquid crystals in a pluralityof directions. FIG. 16A is a top view of a pixel portion, and FIG. 16Bis a cross-sectional view near a liquid crystal layer taken along a lineA-A′ in FIG. 16A. One pixel includes a TFT 1001, a transparentconductive film 1002 serving as a pixel electrode, and a metal film 1003formed thereover. Reference numeral 1004 denotes an opposite substrate;1005, an opposite electrode; 1006, a liquid crystal; and 1007, analignment film A plurality of the metal films 1003 is arranged over onetransparent conductive film 1002. A cross-sectional shape of each metalfilm 1003 is a triangular shape, and liquid crystals in one pixel arealigned in two directions by an inclined surface thereof. Each metalfilm is formed to make a ridge over the transparent conductive film1002. Such a structure is referred to as a so-called MVA (Multi-domainVertical Alignment) method, and wide viewing angle characteristics canbe obtained. Although a cross section of the metal film 1003 has atriangular shape in the cross-sectional view of FIG. 16B, a trapezoidalshape may be employed. Also in the case, liquid crystals in one pixelcan be aligned in two directions by an inclined surface thereof.

FIGS. 17A and 17B show another arrangement example of a metal film by aMVA method. FIG. 17A is a top view of a pixel portion, and FIG. 17B is across-sectional view near a liquid crystal layer taken along a line A-A′in FIG. 17A. One pixel includes a TFT 1201, a transparent conductivefilm 1202 serving as a pixel electrode, and a metal film 1203 formedthereover. Reference numeral 1204 denotes an opposite substrate; 1205,an opposite electrode; 1206, a liquid crystal; and 1207, an alignmentfilm. In FIGS. 17A and 17B, the metal film 1203 forms a plurality ofprojections and each projection has a peak and has a shape like aquadrangular pyramid. Therefore, liquid crystals in one pixel arealigned by the number of inclined surfaces of the projection, namely infour directions. The shape of the projection may be a triangular pyramidor the like other than a quadrangular pyramid, and in that case, liquidcrystals are aligned in three directions. Therefore, with the structureof FIGS. 17A and 17B, wider viewing angle characteristics than FIGS. 16Aand 16B can be obtained.

The example explained in this embodiment mode can be utilized as asubstitute for a slit for giving a specific alignment to a liquidcrystal of a PVA (Patterned Vertical Alignment) method or the like. Byusing the example in substitution for a slit of a PVA method, a step offorming a slit in a transparent conductive film to be a pixel electrodecan be reduced.

In addition, FIG. 18 shows another arrangement example of a metal film.A transparent conductive film 1502 serving as a pixel electrode iselectrically connected to a TFT 1503, and further, a metal film 1501 isstacked over the transparent conductive film 1502. The metal film 1501has a comb shape.

As described above, by an ingenuity of arrangement of a metal film overa transparent conductive film, light shielding can be certainlyconducted and viewing angle characteristics can be improved. Inaddition, the number of forming steps can be reduced because a specialmask for forming a metal film is not necessarily formed.

It is to be noted that each TFT shown in FIGS. 13A and 13B, FIGS. 14Aand 14B, FIGS. 15A and 15B, FIGS. 16A and 16B, FIGS. 17A and 17B, andFIG. 18 has a bottom-gate structure; however, the structure is just oneexample, and another TFT structure can be used. In addition, thisembodiment mode can be freely combined with Embodiment Modes 1 to 4within the range of enablement.

Embodiment Mode 6

As semiconductor devices according to the present invention, camerassuch as video cameras or digital cameras, goggle-type displays (headmounted displays), navigation systems, audio reproducing devices (suchas car audio components or audio components), personal computers, gamemachines, mobile information terminals (mobile computers, cellularphones, mobile game machines, electronic books, and the like), imagereproducing devices provided with a recording medium (specifically,devices provided with a display which can reproduce a recording mediumsuch as a Digital Versatile Disk (DVD) and can display the image), andthe like can be given. FIGS. 20A to 20D and FIG. 21 show specificexamples of the semiconductor devices.

FIG. 20A shows a digital camera, which includes a main body 2101, adisplay portion 2102, an imaging portion, operation keys 2104, a shutter2106, and the like. FIG. 20A shows the digital camera seen from thedisplay portion 2102 side, and the imaging portion is not shown in FIG.20A. In accordance with the present invention, an inexpensive and highlyreliable digital camera with less display failure can be realized.

FIG. 20B shows a personal computer including a main body 2201, a casing2202, a display portion 2203, a keyboard 2204, an external connectionport 2205, a pointing mouse 2206, and the like. In accordance with thepresent invention, an inexpensive and highly reliable personal computerwith less display failure can be realized.

FIG. 20C shows a portable image reproducing device provided with arecording medium (specifically, a DVD reproducing device), whichincludes a main body 2401, a casing 2402, a display portion A 2403, adisplay portion B 2404, a recording medium (such as a DVD) readingportion 2405, operation keys 2406, a speaker portion 2407, and the like.The display portion A 2403 mainly displays image information and thedisplay portion B 2404 mainly displays character information. Thecategory of such an image reproducing device provided with a recordingmedium includes a home game machine and so on. In accordance with thepresent invention, an inexpensive and highly reliable image reproducingdevice with less display failure can be realized.

FIG. 20D shows a display device which includes a casing 1901, a support1902, a display portion 1903, a speaker 1904, a video input terminal1905, and the like. This display device is manufactured by using a thinfilm transistor formed by a manufacturing method described in EmbodimentModes described above for the display portion 1903 and a driver circuit.Liquid crystal display devices, light-emitting devices, and the like aregiven as examples of display devices. Specifically, all types of displaydevices for displaying information are included, for example, displaydevices for computers, display devices for receiving televisionbroadcasting, and display devices for advertisement. In accordance withthe present invention, an inexpensive and highly reliable display devicewith less display failure, in particular, a large-sized display devicehaving a large screen of 22 to 50 inches, can be realized.

In the cellular phone 900 shown in FIG. 21, a main body (A) 901 providedwith operation switches 904, a microphone 905, and the like is connectedwith a hinge 910 to a main body (B) 902 provided with a display panel(A) 908, a display panel (B) 909, a speaker 906, and the like, and it isopenable and closable by the hinge 910. The display panel (A) 908 andthe display panel (B) 909 are placed in a casing 903 of the main body(B) 902 together with a circuit board 907. Pixel portions of the displaypanel (A) 908 and the display panel (B) 909 are arranged such that theyare visible through an opening formed in the casing 903.

As for the display panel (A) 908 and the display panel (B) 909, thespecifications such as the number of pixels can be appropriatelydetermined in accordance with the functions of the cellular phone 900.For example, the display panel (A) 908 and the display panel (B) 909 canbe combined as a main screen and a sub-screen, respectively.

In accordance with the present invention, an inexpensive and highlyreliable mobile information terminal with less display failure can berealized.

The cellular phone according to this embodiment mode can be changed invarious modes depending on functions or applications thereof. Forexample, it may be a camera-equipped cellular phone by implementing animaging element in the hinge 910. Even when the operation switches 904,the display panel (A) 908, and the display panel (B) 909 are placed inone casing, the above-described effect can be obtained. Further, asimilar effect can be obtained even when the structure of thisembodiment mode is applied to an information display terminal equippedwith a plurality of display portions.

As described above, various types of electronic devices can be completedby using any structures or manufacturing methods of Embodiment Modes 1to 8 as the display portion in FIGS. 20A to 20D or the display panel inFIG. 21.

According to the present invention, a conductive film serving as a pixelelectrode and a metal film stacked thereover can be formed using onemask. Further, when the conductive film is disconnected due to leveldifference, the disconnected conductive films can be connected to eachother with the metal film. In accordance with the above, an inexpensivesemiconductor device can be manufactured through fewer manufacturingsteps and a highly reliable semiconductor device can be realized.

This application is based on Japanese Patent Application serial No.2005-301022 filed in Japan Patent Office on Oct. 14, 2005, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a gate electrodeover a substrate; a gate insulating film over the gate electrode; anisland-like semiconductor film over the gate insulating film; aconductive layer over the island-like semiconductor film, the conductivelayer being electrically connected to the island-like semiconductorfilm; a transparent conductive film over the gate insulating film; and ametal film over and in contact with the transparent conductive film, themetal film being electrically connected to the conductive layer, whereinthe transparent conductive film comprises a portion whose thickness issmaller than a thickness of a portion which is in contact with the metalfilm, and wherein the transparent conductive film is a pixel electrodeof a fringe field switching mode liquid crystal display device.
 2. Thedisplay device according to claim 1, wherein part of the transparentconductive film is interposed between the metal film and the conductivelayer.
 3. The display device according to claim 1, wherein thetransparent conductive film is in contact with a side surface of theisland-like semiconductor film.
 4. The display device according to claim1, wherein an outer edge of the metal film is inside an outer edge ofthe transparent conductive film.
 5. An electronic device comprising: thedisplay device according to claim 1; a speaker; a microphone; and aswitch.
 6. A display device comprising: a gate electrode over asubstrate; a gate insulating film over the gate electrode; anisland-like semiconductor film over the gate insulating film; aconductive layer over the island-like semiconductor film, the conductivelayer being electrically connected to the island-like semiconductorfilm; a transparent conductive film over the gate insulating film; and ametal film over and in contact with the transparent conductive film, themetal film being electrically connected to the conductive layer, whereinthe transparent conductive film comprises a portion whose thickness issmaller than a thickness of a portion which is in contact with the metalfilm.
 7. The display device according to claim 6, wherein thetransparent conductive film is a pixel electrode of a fringe fieldswitching mode liquid crystal display device.
 8. The display deviceaccording to claim 6, wherein part of the transparent conductive film isinterposed between the metal film and the conductive layer.
 9. Thedisplay device according to claim 6, wherein the transparent conductivefilm is in contact with a side surface of the island-like semiconductorfilm.
 10. The display device according to claim 6, wherein an outer edgeof the metal film is inside an outer edge of the transparent conductivefilm.
 11. An electronic device comprising: the display device accordingto claim 6; a speaker; a microphone; and a switch.
 12. A display devicecomprising: a gate electrode over a substrate; a gate insulating filmover the gate electrode; an island-like semiconductor film over the gateinsulating film; a conductive layer over the island-like semiconductorfilm, the conductive layer being electrically connected to theisland-like semiconductor film; a transparent conductive film over thegate insulating film; and a metal film over and in contact with thetransparent conductive film, the metal film being electrically connectedto the conductive layer, wherein the transparent conductive filmcomprises a portion whose thickness is smaller than a thickness of aportion which is in contact with the metal film, and wherein thetransparent conductive film and the metal film each have a differenttaper angle.
 13. The display device according to claim 12, wherein partof the transparent conductive film is interposed between the metal filmand the conductive layer.
 14. The display device according to claim 12,wherein the transparent conductive film is in contact with a sidesurface of the island-like semiconductor film.
 15. The display deviceaccording to claim 12, wherein an outer edge of the metal film is insidean outer edge of the transparent conductive film.
 16. An electronicdevice comprising: the display device according to claim 12; a speaker;a microphone; and a switch.
 17. The display device according to claim12, wherein the transparent conductive film is a pixel electrode of afringe field switching mode liquid crystal display device.